Here is a version of the tutorial we posted a couple of days ago, but this time using just a Script Operator to control the particle color based on the particle speed.
http://www.franticfilms.com/software/support/krakatoa/particle_color_by_speed_using_maxscript.php
It turned out to be only 3.6 times slower than the Data Operator version so it is a usable alternative for users who do not have Orbaz’ PFlow Toolbox #3.
Note that you can change the code to make the color dependent on any other particle property or combination thereof, so this approach is much more powerful than using the Particle Age map that ships with Max which can make the color/density dependent only one one property (the particle Age).
To change the density instead of the color, simply write to the particleFloat property instead of particleVector (be sure to enable it using pCont.useFloat = true) and check the other checkbox in the Krakatoa Options operator.
Have fun!